Piezoelectric, ferroelectric, and pyroelectric materials that are capable of converting heat to electricity have been studied for over 20 years. In recent years, ceramics, polymer, copolymer, and composite materials have been produced that have pyroelectric and piezoelectric energy density capabilities of the order of hundreds of kilojoules per cubic meter, or several joules per cubic inch.
Recent lightweight composite electroactive materials have been produced that have weight densities ranging down to one eighth of the density of ceramic piezoelectric materials, and they have comparable energy density capabilities. The new composites also feature magnetoelectric materials that can be poled to generate parallel electric and magnetic fields. Laminated piezoelectric materials have been used as microwave filters and as piezoelectric transformers and amplifiers for many years, however, only recently has a new multiphase composite material allowed production of a Navy hydrophone with a figure of merit 1,000 times that of single phase material.
In recent years, direct drive motors have been constructed with piezoelectric ceramics. The motors are lightweight, compact, and require no field windings. They are capable of providing high torque at low speeds.
Piezoelectric materials may be operated as detectors of vibrational or acoustic energy with high mechanical to electrical energy conversion efficiencies. Conversely, high voltage excitation of piezoelectric materials provides radiated vibrational or acoustic energy, and at higher frequencies, such excitation can provide high power electromagnetic radiation output. The high voltage generating capabilities of piezoelectric materials, as well as their ability to capacitively store electrostatic and magnetostatic energy make the materials attractive as particle acceleration and focusing devices and as energy sources or pumps for klystrons, masers, and lasers. Thinly laminated piezoelectric or composite materials can be impedance matched to diode or transistor varactors for harmonic signal generation and modulation, or to solid state photodetectors, masers, and lasers in compact, rugged packages.
Two major problems with pyroelectric materials are their relatively long cycling times arising from low thermal conductivities, and their lack of long term reliable cyclic operation.
The response time and the cycling abilities of a pressurizing means become important factors when considering pressure activated piezoelectric or composite materials as transceivers for communications or as power transfer and conversion systems.
There are two broad classes of materials that exhibit memory or shape recovery properties on absorbing or radiating heat. They are alloys (or alloy composites) and polymers (or polymer composites) that convert absorbed and radiated heat to mechanical work. Memory materials have been reported that are capable of exerting pressures or forces of the order of 100,000 pounds per square inch of memory material cross sectional area. Polymer and composite memory materials exert forces and perform mechanical work on being cooled, while memory alloys and composites perform mechanical work on being heated.
Memory alloys and alloy composites typically have thermal conductivities of the order of 100 times the thermal conductivities of polymer, composite polymer, or ceramic materials. They are thus capable of much shorter response times and more rapid cycling rates. One memory alloy material has been observed to exhibit a response time of 0.01 seconds. Memory alloys have been reported to have reliable cycling capabilities through ten million cycles. Reliable cycling ability has been reported to be a logarithmic function of strain, and reliable cycling capabilities of the order of a billion cycles at strains of the order of one percent are predicted. Thus, long term reliable cycling capabilities for memory materials of the order of years at cycling rates of 60 cycles per second are predicted.
Typical ceramic piezoelectric compression strains are of the order of 10.sup.-3 or 0.001 percent for optimum operation, while memory alloys operate optimally at strains of the order of a thousand times greater, or a few percent or less. Therefore, a pressure transmitting means or spring material that provides a mechanical impedance match between piezoelectric materials and memory materials, while also acting as a safety means to eliminate inelastic strain in the memory material and thus increase reliable lifetimes, may be incorporated as a separate means, or embodied as a component or compound in ceramic or composite magnetoelectric materials. Memory alloy materials have been produced that reportedly require no spring means to return the material to a desired geometry on cooling, however such materials are not currently available at reasonable cost, and there is no present guarantee that they will be capable of being mechanically impedance matched to electroactive materials.
Both memory materials and poled piezoelectric or magnetoelectric materials may be rejuvinated when performance degrades. Therefore, construction of a preferred form of the invention that allows rejuvination of these materials without disassembly of the invention is advantageous from an economic point of view, and in situations where disassembly is not practicable.
The transition temperature for memory alloys can be specified over a range of the order of 300.degree. C. by specifying the elemental composition of the memory alloy. Thus, a specified transition temperature in the range of -200.degree. to +100.degree. C. can be readily accomplished. The range in temperature over which the memory transition occurs can be of the order of one .degree.C.
A thermodynamic Carnot efficiency of 27 percent has been reported for a mechanical memory alloy machine, where the memory alloy performed work in a flexure or bending mode to turn a wheel. The development of recent new memory alloys and memory composites is expected to allow applications that readily exceed this efficiency when the memory alloy is operated in a pure tension mode and coupled to high efficiency piezoelectric materials. Future advancements in memory composite materials fabrication are expected to provide materials with higher thermal conductivities and emissivities that will further enhance their thermodynamic efficiency and result in more rapid and reliable hot-cold-strain cycling characteristics.